1. Field of the Invention
The present invention relates to a method of producing fluorapatite and a moisture sensitive resistor made of sintered body of the fluorapatite obtained by the same. 2. Description of the Prior Art
Fluorapatite [Ca.sub.10 (PO.sub.4).sub.6 F.sub.2 ; to be described as FAp hereinafter] is a main mineral constituting rock phosphate and used as a phosphor material.
FAp synthesizing methods are roughly classified into a dry method and a wet method.
The dry method includes a step of sintering the raw materials (calcium phosphate and calcium fluoride; calcium pyrophosphate and calcium fluoride; and the like) of FAp at a temperature as high as several hundreds of degrees of centigrade. Therefore, fluorine tends to evaporate easily, and it is difficult to obtain an FAp powder having a uniform composition.
As the wet method, a method involving adding calcium fluoride to a suspension of calcium monohydrogen phosphate and causing the two materials to react to each other, a method hydrolyzing calcium monofluorophosphate dihydrate, and the like are known. However, these methods often accompany by-products. Therefore, it is difficult to obtain pure FAp with these methods.
It is also known as another method of obtaining FAp that, in the process of producing hydroxyapatite, fluorine ions mixed with materials of hydroxyapatite. However, FAp obtained with this method is amorphous.
As still another method of obtaining FAp, a method of mixing calcium fluoride and o-calcium phosphate at a molar ratio of 1:3 and grinding the two materials is known (Kogyo Kagaku Zasshi, Vol. 71, No 9, (1968) p. 1307). However, with this method, the starting materials remain unreacted even after grinding for 8 hours, and it is difficult to cause the starting materials to completely react to each other.
The present invention also relates to a moisture sensitive resistor. A moisture sensitive resistor exhibits a resistance corresponding to the relative humidity of the atmosphere.
Moisture sensitive resistors using ceramics or polymer films are known. However, the electric resistance of a moisture sensitive resistor of this type corresponding to the humidity is high. Particularly, under a low humidity conduction, the electric resistance exceeds the upper limit of the practical measurement range (10M.OMEGA. or less) or the sensitivity tends to be poor even when it falls within this range. There are not many moisture sensitive resistors that can measure a resistance at a high precision in a low humidity range.
Moisture sensitive resistors using apatite are also known. Japanese Patent Publication No. 60-35802 discloses a moisture sensitive resistor using FAp, and Japanese Patent Disclosure (Kokai) Nos. 59-60348 and 59-60350 disclose moisture sensitive resistors using hydroxyapatite. However, the resistances of these resistors corresponding to the humidity are also high. In an attempt to decrease the resistance, some of the cations of apatites has been substituted by other cations (e.g., Li, Na, K, Ag, and Cu). However, even after substitution, the resistances of these resistors are not sufficiently low. In sintered hydroxyapatite, a hysteresis occurs in the resistance when measurement is performed from high to low humidity side and vice versa, resulting in low reliability.